Windward Oahu Flash Flood Event - June 4, 2011

Overview: An unseasonably strong upper level low pressure system produced a very unstable airmass over the main Hawaiian Islands from June 1 through 5. Thunderstorms with frequent lightning and even a few reports of small hail occurred daily across the state. The most significant impact from this system occurred during the early evening hours of June 4 as strong thunderstorms produced intense rainfall and record-breaking stream flow over the windward slopes of Oahu. Flash floods from 5 to 10 inches of rain in a 3-hour period inundated roads and homes and severely damaged agricultural properties from Punaluu to Kailua. Emergency managers also reported several rescues of people from stranded vehicles but fortunately there were no deaths or serious injuries. The rain rates observed during this flash flood event were among the highest values recorded in the last 25 years.

Meteorological Conditions: The month of June normally consists of benign weather conditions with the lowest frequency of flash flood events during the year. Trade winds tend to dominate the lower level flow pattern along with a persistent temperature inversion capping cloud development generally between 5000 and 10,000 ft. However, the presence of the upper level low pressure system over the island chain removed the stabilizing inversion allowing deep convection to develop. Figure 1 provides the geopotential height contour plot at 500 millibars (mb, approximately 18,000 ft above mean sea level) for 800 PM HST 4 June. The map shows the low pressure system centered near Kauai and Oahu. This type of weather pattern is not typically found in June near the Hawaiian Islands. Near the surface (Figure 2), a weakness in the high pressure ridge northwest of the state modified the usual east-northeast trade winds resulting in a slight southerly component in the low level wind field. Figure 3 shows the Lihue sounding consisting of temperature, moisture, and wind observations from 200 PM HST 4 June during the afternoon prior to the onset of heavy rainfall. A notable feature in the sounding is the very cold temperatures aloft. At 500 mb , the observed temperature is -14.3C. The average 500 mb temperature for June is -7.4C with a one standard deviation range from -9.4 to -5.4C. In fact, this same low pressure system two days before produced the coldest 500 mb temperature ever (-16.9C) during the month of June for the Lihue sounding based on records going back to 1973. Compare Figure 3 with the sounding in Figure 4. This sounding, from mid-June several days after the heavy rain event, is more typical of a June sounding over the Hawaiian Islands. Note the temperature inversion at around 6000 ft which suppresses deep cloud development.

Around 400 PM HST June 4, deep convection developed over the Waianae Mountain Range on Oahu. These thunderstorms dissipated close to sunset. However, as the Waianae convection began to weaken, another round of thunderstorm activity fired off at about 500 PM HST, this time over the windward slopes of Oahu’s Koolau Range. A 4-hour loop of Molokai WSR-88D digital hybrid reflectivity radar data starting from 600 PM HST showed that the thunderstorm’s rainfall core (red shaded area) started over the Punaluu area and drifted slowly southeastward over the next several hours before moving offshore near Waimanalo at around 1000 PM HST. Infrared images from the GOES-11 satellite from 400 PM to 1000 PM HST show very cold cloud tops (bright blue shade) from the deep convection appearing over northeast Oahu. The presence of the upper level low over the center of the state is apparent from the counter-clockwise motion of the high clouds (in light gray and light blue). Lightning sensor data from the World Wide Lightning Location Network (WWLLN) registered almost 14,000 flashes around the Hawaiian Islands during the 12-hour period from 300 PM HST 4 June to 300 AM HST 5 June. Of this total, around 5000 flashes occurred over and near Oahu indicating strong intensity of the thunderstorms.

Rainfall Totals: The table below shows the peak rainfall rates during the event (click here for overview map with gage locations; disclaimer). The Fisher-Porter rain gage deep in Waiahole (ID: WHLH1) recorded the highest 15-minute accumulation of 1.73 inches at 745 PM HST, which is a rate of 6.92 inches per hour. The maximum 1-hour accumulation from this gage was 5.48 inches from 730 PM to 830 PM HST while the maximum 2-hour accumulation was 8.62 inches from 645 PM to 845 PM HST. Specifically for the Waiahole gage, this maximum 1-hour total has a 2 to 4 percent annual probability while the 2-hour total has a 1 to 2 percent probability. Note that these accumulations surpassed the maximum values recorded during some of the most notable flash flood events in the last 25 years. The December 1991 Anahola Flood still stands out as the most intense rain event in recent years and the January 1956 Kilauea, Kauai event maintains the record for 30-minute, 1-hour, and 24-hour (38+ inches) accumulations for the State of Hawaii.

Stream Flow: According to the U.S. Geological Survey (USGS), the intense rainfall produced record flows at Kahana Stream, Waiahole Stream, Waihee Stream, and Kahaluu Stream. Please refer to the USGS’ information page at: http://hi.water.usgs.gov/studies/flood/june2011/peak.html for more details. Figure 5 shows a loop of stream height at Waihee Stream and rainfall accumulation at the nearby Waihee Pump in 1-hour time steps from 200 PM HST on June 4 through 1100 AM HST on June 5. The top graph is the stream height while the bottom graph shows rainfall accumulation in inches. The yellow bars in the rainfall graph show hourly rainfall accumulations with the scale on the right-side y-axis. Each dot on both plots show 15-minute data points. Note how rapidly the stream responds to intense rainfall as the rise in stream height and rainfall traces are nearly coincident. Many streams in the Hawaiian Islands behave in this manner. Another point of interest is how quickly the stream recedes to nearly pre-storm levels.

Impacts: Flash flooding caused numerous road closures, mudslides, property damage, and agricultural damage along windward Oahu from Punaluu to Waimanalo. Figure 6 (Disclaimer) depicts areas of reported damage and significant stream flow. The high volume of water at Waiahole Stream, Waihee Stream, Kahana Stream, and Waikane Stream created extremely dangerous situations for motorists at locations where these streams crossed roads. Several people were rescued after their vehicles stalled on roads in the rapidly rising water. Fortunately there were no deaths or serious injuries reported.

Photos:

Waihee Stream at Ahilama Rd.: Note debris line.

Waihee Stream at Ahilama Rd.: Facing downstream near bridge. Note asphalt and other debris.

Waihee Stream upstream from Ahilama Rd.: Note width of flood waters and height of debris line. Prior to June 4, the stream path was closer to the left side of the image. Some of the flood waters jumped out of the channel off the right side of the image and flowed through private property. See next image.

Property upstream from Ahilama Rd.: Facing downstream. Note debris line on equipment. Flood waters jumped out of the stream bank near the spot where the image was taken.

Waiahole Stream: Facing downstream from bridge on Kamehameha Hwy. The stream overtopped the bridge with flow much wider than the normal channel.

Waiahole Stream above Kamehameha Hwy: Chiu Yeung (USGS) points to the approximate peak flow height on the crest-stage gage, about 9 feet above normal levels. The USGS' continuous flow gage shelter, positioned off the right side of the image, was swept away by the flash flood but restored to service a few days later. The background clearly shows vegetation knocked down by the flooding and how much lower the stream level is during normal flow periods.

Waiahole Valley Rd from the USGS site at Waiahole Stream: Flood water exited the stream channel and flowed across this road. Debris lines on the fence across the road indicate the level of how much water flowed here.